Towards phonon photonics: scattering-type near-field optical microscopy reveals phonon-enhanced near-field interaction

Diffraction limits the spatial resolution in classical microscopy or the dimensions of optical circuits to about half the illumination wavelength. Scanning near-field microscopy can overcome this limitation by exploiting the evanescent near fields existing close to any illuminated object. We use a scattering-type near-field optical microscope (s-SNOM) that uses the illuminated metal tip of an atomic force microscope (AFM) to act as scattering near-field probe. The presented images are direct evidence that the s-SNOM enables optical imaging at a spatial resolution on a 10 nm scale, independent of the wavelength used (λ=633 nm and 10 μm). Operating the microscope at specific mid-infrared frequencies we found a tip-induced phonon-polariton resonance on flat polar crystals such as SiC and Si₃N₄. Being a spectral fingerprint of any polar material such phonon-enhanced near-field interaction has enormous applicability in nondestructive, material-specific infrared microscopy at nanoscale resolution. The potential of s-SNOM to study eigenfields of surface polaritons in nanostructures opens the door to the development of phonon photonics—a proposed infrared nanotechnology that uses localized or propagating surface phonon polaritons for probing, manipulating and guiding infrared light in nanoscale devices, analogous to plasmon photonics.     

LaAlO3SrTiO3界面导电性的近场成像

铝酸镧/钛酸锶(LaALO3/SrTiO3)异质界面被发现具有二维导电性并呈现出其他一些有趣的呈展现象.而此前, LaAlO3/SrTiO3异质界面的导电性主要都是通过电学方法进行表征.这里作者首次提出利用散射式扫描近场光学显微镜 (s-SNOM) 对LaAlO3/SrTiO3界面的导电性进行空间成像, 这为研究过渡金属氧化物异质界面体系的物理现象提供了一个新的手段.     

Nanoscale Guiding of Infrared Light with Hyperbolic Volume and Surface Polaritons in van der Waals Material Ribbons

Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale-confined volume modes in thin flakes. On the other hand, surface-confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons—representing typical linear waveguide structures—is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices.